Epstein barr virus induced gene 3 (EBI-3) is a soluble protein that shares structural similarities to the IL12 p40 chain. My preliminary data indicates that EBI-3 and the IL12 p35 chain are highly expressed on Tregs, suggesting that they may pair to yield a novel Treg-specific cytokine. I have also show that Tregs from EBI- 3-/- and p35-/- mice are dysfunctional in vitro. Therefore, the focus of this research proposal is to better understand the importance of EBI-3 and p35 in modulating Treg function. This work will address the following questions: Aim #1 Are Tregs from EBI-3-/- and p35-/- mice dysfunctional in vivo? Because EBI-3 and p35 are required for optimal Treg function in vitro, the necessity of EBI-3 and p35 in vivo will be determined using a variety of animal models. This Aim will address three questions: First, can EBI-3-/- and p35-/- Tregs suppress homeostatic expansion of Teff? Second, are EBI-3-/- and p35-/- Tregs able to rescue mice from inflammatory bowel disease? And, third, is the Teff response to Leishmania infection enhanced in the presence of EBI-3-/- and p35-/- Tregs when compared to wild-type Tregs? Aim #2 will focus on determining biological activity of the EBI-3-p35 pair?. First, we will need to verify IL34 protein expression and secretion in Tregs and the lack of protein made by Teffs. The remainder of this aim will focus determining if ex vivo ectopic expression of IL34 can confer a regulatory T cell phenotype to effector T cells and whether overexpression of IL34 in vivo can induce Treg development. Aim #3 will address the question, what is the mechanism by which IL34 functions? Preliminary data suggests that IL34 is expressed exclusively by Tregs and in vitro functional data indicates that IL34 may be necessary for optimal Treg activity. However, the mechanism by which IL34 acts is unknown. Therefore, to determine the function of IL34 we will need to generate a recombinant IL34 protein. This recombinant protein will be used to address the following questions about the function of IL34. First, does IL34 enhance Treg proliferation or activity? Second, can IL34 directly inhibit Teff proliferation or induce apoptosis of Teff? Third, does IL34 interfere with IL12, IL23, or IL27 activity as a mechanism of action? And, fourth, what genes are induced by IL34 within the Teff or Treg that might facilitate IL34 function? Tregs are important suppressors of the immune response. Their enhancement is beneficial for autoimmune diseases and their depression can support tumor immunity. To utilize Tregs control or prevent disease, it is critically important to understand the proteins that mediate their function. We will investigate IL34 as a novel Treg-specific protein that may be a therapeutic target for autoimmune diseases and/or cancer.